Conventional residential water heaters operate by constantly maintaining a fixed volume of heated water within a storage tank. These water heaters allow unheated water to enter an insulated storage tank until the tank contains the fixed volume of water. The water inside the tank is heated either through the combustion of a gas or by electrical resistance means. Since the heated water will float above any unheated water, a hot water outlet is typically located at the upper end of the storage tank. This hot water outlet allows water to exit the storage tank and be directed to appliances that require its use.
The problem with such conventional water heaters is their high consumption of energy. Because a fixed volume of heated water must always be maintained, the water in the storage tank must continuously be heated so as to maintain its temperature. Although insulation of the storage tank is partially effective in reducing the heat loss encountered by the stored water, additional energy is still required to keep the water at its hot temperature.
Because of the costs associated with this high consumption of energy, it is important that a water heater maintains only as much heated water as demanded by its users. The hot water demand of a household may depend on a variety of factors, such as the month of the year, or the number of people within the household. The demand can change, for example, as children grow up and leave the household or when members of the household leave to go on a vacation. It is therefore desirable to have a water heater in which the volume of water heated and stored may be increased or decreased in response to changes in the hot water demands of its users.